The Wnt family of secretory glycoproteins is one of the major families of developmentally important signaling molecules which play important roles in embryonic induction, generation of cell polarity, and specification of cell fate. Both genetic and biochemical studies indicate that frizzled (Fz) and LRP5/6 are co-receptors for transducing canonical Wnt signaling that eventually leads to the stabilization of β-catenin and regulation of gene transcription through transcription regulators including lymphoid enhancing factor-1 (LEF-1) and T cell factors (TCF). Wnt signaling is also regulated by a number of naturally occurring antagonists that include Dickkopf (Dkk) molecules. The first Dkk (Xenopus Dkk-1), was initially discovered as a Wnt antagonist that plays an important role in head formation. To date, four members of Dkk have been identified in mammals. However, only the first two members (Dkk1 and Dkk2) have been well documented to function as antagonists of canonical Wnt signaling. Both Dkk1 and Dkk2 antagonize canonical Wnt signaling by simultaneously binding to LRP5/6 and a single transmembrane protein called Kremen. It has been further demonstrated that the second, but not the first, Cys-rich domains of Dkk1 and Dkk2 inhibit canonical Wnt signaling.
A myriad of evidence demonstrates that an increase in LRP5/6-mediated canonical Wnt signaling leads to an increase in bone mass. Loss-of-function mutations in LRP5 are responsible for human osteoporosis-pseudoglioma syndrome (OPPG), an autosomal recessive disorder, while putative gain of function mutations, including the Gly171 to Val substitution, are associated with human high bone mass (HBM) phenotypes. In addition, mice in which the LRP5 gene was inactivated by gene targeting showed phenotypes similar to those of OPPG patients, and the transgenic expression of LRP5G171V in mice resulted in HBM. Moreover, mouse primary osteoblasts showed reduced responsiveness to Wnt and low proliferation indices in the absence of LRP5, and canonical Wnts or activated β-catenin stimulated the canonical Wnt signaling activity and induced the production of an osteoblast marker alkaline phosphatase (AP) in osteoblast-like cells. The finding that inactivation of the Wnt antagonist sFRP1 enhances trabecular bone accrual further supports the idea that canonical Wnt signaling enhances bone formation. Dkk1 is expressed in differentiated osteoblast cells and osteocytes and the G171V mutation in LRP5 may cause the HBM phenotype by attenuating the antagonistic effect of Dkk1 on canonical Wnt signaling.
Itasaki et al. described a new Wnt antagonist called WISE. WISE appears to be a context-dependent regulator of Wnt signaling; it may inhibit or stimulate Wnt signaling in different assays in Xenopus. WISE was also shown to bind to LRP6 and compete with Wnt8 for binding to LRP6. WISE shares 38% amino acid identity with sclerostin, the gene product of SOST. Loss of function mutations of SOST are responsible for an autosomal recessive sclerostin skeletal disorder. Previous studies have shown that sclerostin was highly expressed in osteocytes and that it might act as a bone morphogenetic protein (BMP) antagonist, but another study suggested that sclerostin might not be a functional BMP antagonist and speculated that it might modulate Wnt signaling. In this report, we now clearly demonstrate that sclerostin can bind to both LRP5 and LRP6 and act as a Wnt antagonist. Because sclerostin expression occurs after peak Wnt7b expression during the osteogenic differentiation, the reduction in sclerostin-mediated antagonism of Wnt signaling contributes to the increases in bone mass associated with SOST.